An organic electroluminescence (referred to below as [organic EL]) display device is provided with an organic EL element in each pixel and displays an image by controlling light emittance. An organic EL element has a structure in which a layer (referred to below as [organic EL layer]) including an organic EL material sandwiched between a pair of electrodes wherein one is an anode electrode and the other is a cathode electrode. In an organic EL display device, one of the electrodes is provided as a pixel electrode for each pixel and the other electrode is provided as a common pixel electrode bridging a plurality of pixels and is supplied with a common potential. In an organic EL display device, the light emitted from a pixel is controlled by supplying a potential of the pixel electrode for each pixel with respect to the potential of the common pixel electrode.
In an organic EL element, it is necessary for at least the electrode on the side which outputs light emitted by an organic EL layer to have translucency. Although a metal semi-transparent film formed by an ultrathin film is sometimes used a translucent electrode, in many cases, the electrode is formed using a transparent conductive film material represented by ITO (indium added with tin oxide). For example, in an organic EL display device, in the case where the light emitted by an organic EL element in a pixel is output from a common electrode side, a reflective surface of a metal is provided on a pixel electrode and the common pixel electrode is formed using a transparent electrode material.
A transparent conductive film material is a metal oxide and becomes problematic since its resistance is high compared to a metal material such as aluminum. In an organic EL display device, because a common pixel electrode is formed as an integrated electrode across roughly the entire surface of a pixel region, when the area of the pixel region becomes large, that is, when the size of the screen becomes large, the effects of the resistance of the common pixel electrode formed with a transparent conductive material cannot be neglected. Image quality defects called shading occurs when there are common pixel electrode resistance effects. Shading is a phenomenon whereby when a drop in voltage due to an increase in electrode resistance can no longer be neglected, variation in luminosity occurs within a display screen and display quality deceases. When shading occurs, defects such as drop in luminosity of a central region compared to an exterior side region of a screen become visible.
In order to solve this problem, providing auxiliary wiring in a pixel part in order to reduce the resistance value of a common pixel electrode has been proposed. In order to reduce apparent resistance of a common pixel electrode using auxiliary wiring, it is necessary to connect the auxiliary wiring with the common pixel electrode within a region of a pixel part. In this case, since forming auxiliary wiring together with a photolithography process after forming an organic EL layer is difficult because this causes the organic EL layer to deteriorate, usually it is preferred that the auxiliary wiring is formed first.
For this reasons, a method for connecting a common pixel electrode and auxiliary wiring by connecting the common pixel electrode and auxiliary wiring provided on a lower layer thereof via a contact hole, and a method of physically removing an organic EL layer and forming a contact in the case where the organic EL layer exists between the common pixel electrode and auxiliary wiring have been proposed.
For example, a structure is disclosed in Japanese Laid Open Patent No. 2008-135325 in which auxiliary wiring is formed in advance above a bank which encloses a pixel electrode and the auxiliary wiring and common pixel electrode are connected above the bank. The structure disclosed in Japanese Laid Open Patent No. 2008-135325 is formed by providing auxiliary wiring on a lower layer, center layer and upper layer wherein the upper layer protrudes further than the center layer and the lower layer is hidden beneath the peak of the upper layer. Even when an organic EL layer is deposited after a three structure auxiliary wiring is formed, the organic EL layer is not deposited on the lower layer due to the peak of the upper layer. When a common pixel electrode is formed above an organic EL layer using a sputtering method and a cover layer is wrapped around the lower layer of the auxiliary wiring, it is possible to connect the common pixel electrode with the auxiliary wiring even if a contact hole is formed.
As another example of connecting a common electrode with auxiliary wiring, an organic EL device is disclosed in Japanese Laid Open Patent No. 2005-268024 in which a metal film is formed on upper surface of a bank and this is used as the auxiliary wiring of the common pixel electrode. In Japanese Laid Open Patent No. 2005-268024, when a metal film which acts as an anode protective layer using a bank having a T shape or reverse taper shape cross-sectional shape as a mask is formed above an anode, a metal layer formed above the bank is simultaneously used as auxiliary wiring. Since the metal layer is separated from a metal layer deposited on a lower layer due to the shape of the bank, this is used as the auxiliary wiring of the common pixel electrode.
However, since the auxiliary wiring disclosed in Japanese Laid Open Patent No. 2008-135325 and Japanese Laid Open Patent No. 2005-268024 both contact with a common pixel electrode on a flat surface, a fixed area is taken up within a region of a pixel part. As a result, the area which can be used as a pixel is reduced and the aperture ratio is reduced.
When trying to achieve high definition of a pixel, it is necessary to reduce pixel pitch and increase the number of pixel per unit area. At this time, it is necessary to perform a miniaturization process to provide auxiliary wiring above a bank and it is difficult secure an area for contacting the auxiliary wiring with a common pixel electrode even if the auxiliary wiring can be miniaturized.